In this Ph.D. dissertation, issues regarding Design for Testing (DFT) of portable safety critical applications are concerned. The international standards define as safety critical applications every application that preserves, supports or affects our day-to-day life, either it reflects to one person or the whole society. The majority of portable safety critical applications are met in the medical devices field. Examples of such devices are the pacemaker, a medicine infusion pump etc. The standards dictate that design and development of such systems must guarantee that their operation lies between strict safety margins.
The main goals of system design, targeting the medical devices market, are reliability and on time fault detection. High reliability of a system is achieved by means of material quality that preserves its behavior even under a hostile environment e.g. high radiation. A common approach, mainly due to the cost of safe materials against EMI, is the multi-channeled architecture of the system. This technique is also a primitive and efficient form of fault detection, although it introduces high requirements in integration area and high power dissipation. Other commonly followed techniques are software (benchmarks) and hardware redundancy (BIST units). However all the latter techniques present either low reliability or don’t manage to meet the time constraints imposed by the standards on fault detection.
The research activity which was developed during these years and is presented in this dissertation is targeting the on-line concurrent testing. For the needs of the research, a system design methodology was developed, covering design steps from the very first, which is specification definition up to the system level design. Every circuit which was designed is based on encoding and decoding of the information. All the techniques and encoding schemes that were used are presented, in terms of design level abstraction, and further analyzed.
In the bounds of the proposed methodology a fault model is initially introduced for the entire design flow of development and design of a safety critical system. Furthermore, a deterministic measure, which is enriched with the activity probability, is defined to facilitate evaluation of a circuit’s reliability. The FSES tool (Fault-Secure Evaluation Script) was developed for the evaluation process automation which addresses special issues of concurrent testing, while its novelty is the concern of the applications characteristics to guarantee that the circuit’s behavior and design meet the specifications. Thus, encoding scheme selection is no more an adhoc issue but is driven by experimental and precise values.
A set of parametric self checking circuit generators was developed, in terms of encoding scheme and bit-width. This type of circuit allows fault detection in near real-time. Thus, the whole spectrum of basic circuits is offered to a designer to develop the system in the first level of design, the Circuit level.
In the Register-Transfer (RT) level, every structure that can be used is presented. Design and interconnection guidelines are offered in order to preserve the system’s level of reliability. A variety of Low-Power Design techniques are evaluated and modified thus no violation of the concurrent testing constraints is observed.
Every unit that supports operation of the system at the System level is referred and mechanisms are proposed to attain and even increase system’s observability. Special mechanisms that continually monitoring system’s tasks fault-free execution are presented. In case of a fault occurrence, depending on the degree of operation affection, they produce either a sequence of warnings or a special mode of safe resetting and system restart is activated. At this level, a technique to ensure code execution consistency is proposed, so that unpredicted changes of the program’s flow to be detected.
Due to the requirement of portability, in fact extended autonomy, this dissertation concerns, wherever possible, the enrichment of the DFT method with elements of the Low-Power Design method. However, it is showed, from the very first chapter, that these methods of designing a system present a controversial nature. The techniques of both methods that meet the specifications are exhibited while design guidelines of do and don’ts are offered. Applying the presented methodology, a gain of even 40% in power dissipation was achieved for certain circuits.
Finally, the methodology was applied on the development of a prototype mobile medicine infusion pump system that infuses insulin to diabetics. A programmable microcontroller was developed, that includes a programmable shifter, a multiplier and a complete ALU. It is TI MSP code compatible, for code re-use issues. It can manage 128 Kb and 256 peripherals, while it includes a PIC. The system embeds a sophisticated PID controller, which was designed according to the proposed methodology, to drive the pumps motor and ensure normal flow of the medicine.
The prototype was integrated in Frauhoffer-Institute using the AMS 0.6 um. A yield of 90% was achieved due to the a priori correct by-construct approach that was followed. A system that embeds fault detection by means of software and hardware redundancy was used as a reference vehicle. The comparison of the two systems’s exhibited a 36% decrease of power dissipation of the prototype than the reference system. Additionally, fault coverage in the prototype exceeded 98% which is the lower bound imposed by the standards.
Concluding, this dissertation offers design solutions for portable safety critical applications considering from the very beginning the application’s requirements. Thus, a high safety level of operation is achieved in contrast to the commonly used techniques which are mainly applicable to general purpose applications. The benefits are not restricted to cost and power savings but it is mainly the safe operation of a system at every level of abstraction by default.
